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What is this high flying aircraft over Pennsylvania?
Yesterday afternoon I photographed a Boeing 777-300ER flying over my home, according to radar data it was at approximately 32,000 ft. In the photograph there is another aircraft present, it is much smaller and higher in altitude (40,000-50,000 ft) and it is a small aircraft, possibly a drone or military. It did not appear on any flight tracking sites. I usually see military traffic flying in that particular route and heading. I have numerous "raw" images of this particular aircraft.

I originally though it may be an L-39 although that particular aircraft does not have a T tail design and I think that it would struggle at that altitude. The photographs were taken at 1545 EST over the LVZ VOR.
The aircraft was flying an almost perfect east to west heading usually reserved for military traffic. One interesting note is that it was not leaving a contrail. I routinely photograph B-52's, tankers, and fighter aircraft transitioning over my home at or about that altitude and they almost always leave contrails.

welcome to aviation.SE. If you want help identifying an aircraft, you will have to provide your data here in the open, we don't do anything via other means.

Also note that after enlarging it appears to be a single "inline jet" engine configuration similar to a U-2's fuselage? Thanks Joseph

I asked a co-worker who flew U-2 and he says that to his knowledge all the wings used have a taper on the rear.  But he has not flown ALL the variants.  Also he points out that there are no visible pods for sensors, which is common with operational flights.  Plus the paint schemes on all the U-2 that he has seen as operational are low reflectivity paints, and would not appear as in the photo.  How about giving us more data on your photo acquisition details?

as you seem to have created 2 accounts, please have a look on how to merge them and regain control over the question: https://aviation.stackexchange.com/help/merging-accounts

please use the [edit] functionality, if you want to add information.

I think when you wrote "1545 EST" you meant "15:45 EDT (UTC−4)"

@ymb1 That's nice to combine the image. Although, the arrowhead needs to be pointing to the bigger image.

Using the location you gave, I tracked back aircraft in that area at that time, and found a scenario that fits with your photo:

(Source: Flightradar24)
So, judging by that, the jet in question is actually a Learjet 31, as said by John K.

Here are some blueprints, and the dimensions are similar:

Wow well done!!

I did a bit of pixel measuring.  In the image, the Boeing 777-300ER is 87 pixels long;  the Learjet is about 12 pixels long.  A 777-300ER in 75 m long;  a Learjet 31A is 15 m long.  Putting all of these numbers together, we can infer that the Learjet was roughly 1.5 times farther from the camera than the Boeing.  That's not too far off from the reported altitudes of 32,000 and 45,000 feet, particularly if the OP was at a higher elevation.

any idea why the plane didn't leave a trail?

@Manuki Contrails require specific atmospheric conditions that may be present at one altitude but not another.

(I should note that my pixel values above applied to the size of the original image on my screen.  The image has since been changed, and I apparently wasn't viewing it at the highest possible resolution.  But the basic logic still holds.)

I'd say it's a corporate jet. Corporate jets normally play between 40-55000 ft, above the bulk of the airline traffic down in the 30s, so this is a perfectly normal sight.
Based on the wing planform with straight trailing edge and swept leading edge, and what looks like a T tail and ventral fins, I'm going with Lear 45 or a similar Lear variant (Service ceiling 51000 ft).  The viewing aspect doesn't look like from directly below so the engines won't stick out very clearly.

And, if you look carefully at the OP's photo, you can actually see a hint of something just behind the wings, in exactly the right place for the engines on a Learjet.

What is the reason for this separation of flight levels?

Corporate airplanes just tend to have higher service ceilings, in the 50k range, while most airliners are in the low 40ish range, and generally operate between 30 and 40.  It's unusual to operate right at your service ceiling - you have no surplus energy margin to speak of and it can take forever to get there - unless you are unusually light.  You have to monitor your speed very carefully.  If you slow down too much you have no choice but to descend.

@JohnK: IIRC, the most fuel-efficient altitude is around the tropopause, so higher than ~36k isn't better.  Just because corporate jets can fly higher doesn't fully explain why they routinely do.  But since they can while airliners can't, corporate jets fly higher to stay out of the way?

I'd say it's mainly no traffic, and at least in the middle latitudes, you can fly over anvils with decent margin and avoid most CAT, which is usually associated with jets that reside at the tropopause at frontal boundaries.  If you're crossing the Atlantic, you can go straight across without having to run in the NA tracks which top out at 41.


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How is smoke detected in the cabin when flying without passengers?
The Boeing website states that smoke detection devices are not present in the passenger cabin as the passengers/cabin crew are able to detect the smoke:

A dedicated smoke detection system is not required in the occupied volumes of the main cabin due to the ability of passengers and the cabin crew to recognize smoke.

How would smoke be detected in the cabin when flying without passengers or cabin crew? As an example, this could be the case on a ferry flight where no one in the cabin may be present to detect smoke.

I don't think this is common for most commercial jets so I am not putting it as an answer, but I worked on a special freighter type that required all power to be shut off to the supernumerary if there was no crew member in it.

When travelling without passengers we we would open all the lavatory doors, as they do have smoke detectors.

Based on smoke detector testing I've seen, I doubt that would be very effective at detecting a fire while still small enough to be put out by a portable extinguisher.

The threat for a cabin compartment smoke or fire comes from paying passengers & baggage.
On a ferry flight, you have neither, so the threat is greatly reduced or eliminated.
The FAA authorizes ferry permits for only 1 or a few  segments to get an aircraft to a facility where maintenance can be performed, these ferry permits take into account the risks involved and thus why no revenue cargo / passengers can be carried.

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Can I share ACARS messages online?
ACARS messages are transmitted via radio just like normal ATC communications are. In the UK, for example, it is illegal to listen to and broadcast ATC chatter. Does the same apply to ACARS? If an ACARS message is sent from a ground station to an aircraft, say an ATC departure clearance, and I read it using a VHF receiver, would that be illegal? Can I share it online?
I understand the reason we can't find Live ATC for Canada, UK, Germany etc. is because of the local laws. But from what I can see these laws are related to voice transmissions, not text.

As I'm sure you're aware, this portion of the Wireless Telegraphy Act is essentially unenforced for casual and personal airband scanning, with many airports, and air shows openly promoting and publicising their frequencies for enthusiasts.
However, I'm not at all sure where you've gotten the idea that the law refers to voice. There are two main elements of the Wireless Telegraphy Act to be aware of:
S.48:
https://www.legislation.gov.uk/ukpga/2006/36/section/48/enacted

A person commits an offence if, otherwise than under the authority of a designated person he uses wireless telegraphy apparatus with intent to obtain information as to the contents, sender or addressee of a message (whether sent by means of wireless telegraphy or not) of which neither he nor a person on whose behalf he is acting is an intended recipient

or

he discloses information as to the contents, sender or addressee of such a message.

As you can see, nothing in the act limits itself to voice, or even analogue communications. Ironically, if anything, the wording is anachronistic with the Telegraphy originating in the era of Morse transmissions but the legal intent is clear: Intercepting communications of any sort, not intended for you, is an offence. In fact, it is even clearer that it applies to ACARS than to voice, because, being a text-based protocol, ACARS is actually telegraphy, while voice radio is not for most purposes.
Either way, to clearly answer your question, ACARS and any other digital communication is covered identically under the act

So in theory the ACARS messages needs to be coming from a ground station based from a country where these laws don’t exist? E.g. an aircraft flying in the US getting information about a UK airport. The ground station would be based in the US.

@user38116 Pretty sure it depends on where you are when intercepting the radio transmission, not from where it was sent or to whom it was intended.

Interesting, so you can intercept messages in Ireland for a message destined to a UK aircraft with information relating to the UK. I wonder how it would work in terms of ADSC, I assume it would work similarly. So if your satellite receiver is in Ireland then you’d be safe to intercept and share them.

Related regulations are local, but are the implementation of obligations agreed by all member States of the International Telecommunication Union (ITU), an agency of the UN, so most countries have similar obligations, US might be a notable exception, in spite of being an ITU member.
For ITU, a radiocommunication is a communication by radio, and a communication is anything:

Any transmission, emission or reception of signs, signals, writings, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems

Members have signed the Convention of the International Telecommunication Union which introduces the Radio Regulations (RR), RR say:

Secrecy
17.1 In the application of the appropriate provisions of the Constitution and the Convention, administrations bind themselves to take the necessary measures to prohibit and prevent:
17.2 a) the unauthorized interception of radiocommunications not intended for the general use of the public;
17.3 b) the divulgence of the contents, simple disclosure of the existence, publication or any use whatever, without authorization of information of any nature whatever obtained by the interception of the radiocommunications mentioned in No. 17.2.

So each member has to implement laws prohibiting the interception of radiocommunications, and the divulgence of the content, except for public broadcast.
Usually this is done be delivering licenses to authorized users or organizations, and establishing a general obligation of confidentiality for citizens, regarding communications for which they are not an authorized recipient. This is what UK does in the Wireless Telegraphy Act of 2006, as well explained by @Dan in his answer. In addition, Ofcom, the regulating body states:

Using spectrum without a licence or the benefit of an exemption, or in breach of the conditions of a licence or an exemption, is a criminal offence.

If you don't have a license to listen or transmit on the aeronautical frequencies, you are not allowed to listen. Listening and then disclosing the content is a breach of the general confidentiality obligation.
Note in EU (and possibly still in UK) scanners and receivers/transmitters for specific non-public bands can be sold, this is explicitly allowed by EU regulations, on the count of free circulation of goods.

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On the A321neo, what determines the 'time to land' when an Additional Center Tank System Fault is triggered?
On the A321neo, the QRH has a procedure in the case of a ACT (Additional Center Tank) System Fault (Found in the QRH under Abnormal and Emergency Procedures, Fuel).
When such a fault occurs, you must land within the stated time in the following table, depending on the number of refueled ACTs at dispatch. My understanding is that ACTs are refueled in the following order: AFT1, AFT2 and FWD. They are then used, in flight, in the reverse order: FWD, AFT2 and AFT1.
According to the table, if you have refueled just 1 ACT (which implies you only refueled AFT1), you need to land within 120 minutes of the fault occurring. If you refueled all 3 ACTs, and AFT1 ACT XFR fails (which implies FWD and AFT2 ACTs are empty, due to the order of fuel consumption), you need to land within 200 minutes of the fault occurring.
Where does this difference of 80 minutes come from? In both scenarios, the only ACT with fuel in it would be AFT1. So why would you be given more time to land the aircraft in the latter scenario?

If AFT1 ACT XFR fails, it does not mean that also AFT2 and FWD XFR are not working, indeed you can still use them in manual mode (even if not in the normal order). However, I don't think that the flight time depends on the fuel quantity itself, because the procedure requires you to use only wing tanks (so the fact that AFT2 and FWD are still usable are not relevant for the flight time).

The reason for that flight time is due to the CG limitations.
I don't have data from the 3 ACTs, but I found this revision of the AFM document of the A321 with 2 ACTs from FAA regarding this (link). Shortly, the flight time roughly keeps the ZFCG (Zero Fuel Center of Gravity) inside the allowed region. This region depends on how many ACTs are not empty.


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Why do I hear airplanes' engine sounds in the sky?
I've been wondering all my life about an issue related to aviation. I see airplanes, large airplanes they look commercial in nature but I'm not sure, flying in the northern side of where I live and where the sea is located. I don't live next or near an airport, there's only one airport located far away from where I reside, about 20 or more Km. But I always hear airplanes flying, the sound of the engine is like the sound of thunder. Sometimes I hear the sound at night, sometimes I hear it at evening and I always ask myself, why do they fly here and at a very low altitude and the airport is just so far from my location? Though, I see the airplanes themselves, so I'm sure they're flying over the sea I guess.

This question will be more answerable if you can provide the approximate location (e.g. town) of your residence.

Hi, welcome to Aviation.SE! This is a good question, but before we can give you a good answer we probably need more information. If you could tell us where you are located or your location relative to the nearby airport that would be very helpful.

Planes descent for landing following a slope at about 5%. At 100 km from the runway they would be at about 5 km height, and at 20 km, they are at 1 km. This is indeed low, as in cruise they fly at about 10 km. If you can tell us what is the airport nearby, we may provide you the actual path aircraft follow to land.

20 km is not that far for an airliner. At the speeds they move over ground, even in approach phase where they move slower than in cruise, they are few minutes from landing or after taking off.
Depending on prevailing winds, the airport might be using an approach path or another. That means some days you'll see airliners overflying you non-stop, while other days you might not see a single aircraft.

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Do aircraft depressurise when taking off from a high elevation airport?
A jet aircraft generally maintains a cabin pressure of 7000-8000 ft during cruise. If the aircraft departs from a high elevation airfield, such as Quito (approx. 10,000ft), will the aircraft depressurise down to 7000-8000 ft?

Does this answer your question? https://aviation.stackexchange.com/questions/11456/how-does-an-airliner-deal-with-cabin-altitude-at-an-airport-above-8000-ft?rq=1

Changing the cabin pressure from 10,000 ft to 7,000 ft would be pressurizing, not depressurising.


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What happens if you preselect your destination QNH during cruise?
I've noticed some pilots preselect their destination QNH on their side of the FCU. In order to preselect the QNH, you need to momentarily switch out of STD which made me wonder:
Assuming your side is switched on for altitude reporting (on the ATC/TCAS panel):

Will ATC receive a different altitude while the pilot is preselecting their QNH?
Does TCAS use this barometric altitude during interrogations? So could this trigger a TCAS if the QNH difference to STD is drastic enough?

I know that it usually on takes a few seconds to do, but could be longer if the QNH difference is big enough or if the pilot is distracted.

Mode-C transponder reporting is done using QNE barometer settings, regardless of the QNH setting by the pilot. This precisely avoids the problem that you mention. The goal of the transponder is to avoid other airplanes, so a common setting like QNE is the logical choice. Maintaining enough height above the ground is the pilots' responsibility, so QNH is only for the pilots display.

Note that the barometric pressure setting does not affect the altitude transmitted by the transponder (which is indeed QNE based),but a Mode-S transponder can provide the barometric pressure setting if the radar is asking for it (It is in BDS register 4,0). The altimeter setting is also part of the ADS-B transmissions.

@DeltaLima Neither is used for separation purposes though but is of course relevant for terrain warning. The real odd one out is FLARM which AFAIK uses GPS altitude.

FLARM uses GPS height indeed, but that is understandable. That way it can be a stand alone instrument that doesn't need static air pressure, which makes it easier to install. Barometric pressure setting is not use  for separation purposes directly, but can cause alerts in ATC systems if it is not set to the correct value for the airspace the aircraft is flying in.

@DeltaLima Right! My club flies in a TMZ so I've never understood why FLARM doesn't just use pressure altitude since "obviously" everyone has a transponder too.

ATC will not see a different altitude. The radar ATC is known as secondary surveillance radar or SSR, where transponders return a pilot set 'squawk' code and the pressure altitude. These transponders have their own altimeters and report the pressure altitude, which is not adjustable to the pilot.

Do you mean QNE instead of QFE?

That was in error @user38116, transponders send the airplane's pressure altitude, not QNH, QNE or QFE.

Note that the barometric pressure setting does not affect the altitude transmitted by the transponder (which is indeed QNE based),but a Mode-S transponder can provide the barometric pressure setting if the radar is asking for it (It is in BDS register 4,0). The altimeter setting is also part of the ADS-B transmissions.

If it is in fact part of the ADS-B transmission, would ATC not be alerted by a deviation in altitude?


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How beneficial are flaps when landing with a tailwind?
I understand that flaps are devices used to increase the amount of lift and drag. When landing with a tailwind your groundspeed will be higher. Therefore, flaps would help reduce this groundspeed, logically allowing for a lower touchdown speed.
Should flaps always be applied with a tailwind? Are there reasons not to use flaps when landing with a tailwind?

Forget about tailwind/headwind for flap use.  Flaps allow landing with a lower airspeed.  And you always want a lower airspeed.  The wind itself is irrelevant except that your groundspeed will be higher or lower depending on where the wind is, and which you always want to be slow as possible.  On light aircraft you want to avoid tailwind components over 5kt. Jets are usually certified, takeoff and landing performance wise, with a max tailwind component of 10-15 kt. But in any case you are still using normal landing flap to have the minimum airspeed, giving the minimum groundspeed for a particular condition.
The time when you may want to land with less than landing flap is when the winds are strong and very gusty (and you will be landing into wind if you aren't crazy) and you want to have a bit more energy with a bit less drag (so the aircraft conserves more energy with its inertia) during the landing flare to cope with sudden small airspeed variations from gusts, which can drop you on the runway hard before you can react if you have the flaps all the way out.

On a windy day there is a typically a wind gradient, meaning that the wind speed increases with altitude.  On normal into-the-wind landings, the wind gradient tends to exacerbate any accidental "ballooning" (meaning that if aircraft accidentally rises during the landing flare, it tends to keep rising, till it eventually ends up in a very low-airspeed condition.)  Especially in gusty conditions.  Flaps tend to aggravate this.  When landing downwind, the wind gradient tends to reduce any accidental "ballooning" tendencies during the landing flare.  On the other hand, downwind landings involve high groundspeeds which can tend to exacerbate steering problems, especially in tailwheel aircraft.  Flaps reduce the airspeed, and therefore the groundspeed, at touchdown.
As a practical matter, when playing around with multiple upwind and downwind touch-and-goes back-to-back in windy conditions with a radio-controlled model airplane (most pilots would not be interested in doing such a test with a "real" plane!), in a situation where runway length was not a limiting factor and the criteria of interest were a smooth gentle contact with the ground followed by a highly controllable run on the wheels before liftoff, I've found that the best results were generally achieved by using flaps when landing downwind and not using flaps when landing upwind.

Consider that if there were no wind gradient, and if wind gusts are equally sharp-edged on their trailing edge as on their leading edge-- i.e. if the gust decreases fully as rapidly as it begins-- then as far as the dynamics that place in the air (before the wheels touch) are concerned, flaps would be just as disadvantageous when landing downwind in strong gusty wind as when landing upwind in strong gusty wind.  Two different "ifs" there, so it's hard to isolate the most important factor.

Related: https://aviation.stackexchange.com/questions/104914/what-are-all-the-ways-that-wind-velocity-can-affect-a-paraglider-canopy-before, specifically "I've heard that gusts are sharper on the upwind side due to how pressure waves work."


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Why is 5 seconds the time to respond to a TCAS RA?
With TCAS, the time to respond to an RA is 5 seconds (2.5 seconds for a reversal RA). The FCOM states that the TAU (estimated time) before the aircraft intruder reaches the "Closest Point of Approach" is 25 seconds (40 seconds for a TA). Why is it then, that pilots are required to react within 5 seconds? Is it simply to add a margin of safety (an extra 20 seconds) before an intruder aircraft comes dangerously close to collision?

If the pilot reacts at impact it is too late to avoid collision
If the pilot reacts one second before impact it is too late to make any significant altitude change and to avoid collision.
If the pilot reacts two... you get it.
The TCAS algorithm calculates with manoeuvres of approximately 20 seconds to create sufficient vertical spacing to avoid a collision. An extra 5 seconds margin is added for the pilot to initiate the manoeuvre after the RA is given.
So there are 5 seconds for safety margin, and 20 seconds for the laws of physics.

I believe that 20 seconds is also so people in the aircraft don't die from the maneuver. So an otherwise-equivalent flight with no passengers could potentially have a smaller window. An A320 is about 40 feet tall from the bottom of the engine to the top of the vertical stab. Normal climb is 2000 ft/min, or 33 ft/s. Rapid ascent/descent can be much faster. With both pilots reacting, it's twice as fast. I.e., it doesn't take close to 20 seconds for the aircraft itself to get out of danger altitude.

There was a Japanese flight some years ago where the pilot reacted at the last seconds (estimated about 2 seconds before impact), narrowly avoiding a collision, but multiple passengers and flight attendants were injured as a result. https://en.wikipedia.org/wiki/2001_Japan_Airlines_mid-air_incident (It was a 747 there, but I don't think the performance is that different for my point.)

Why not more than 5, or more than 25 s total?  I'm guessing because false positives are more likely if you have to predict a collision too far in advance, and you want TCAS to be treated with urgency, not a common occurrence.

When you get an TCAS climb or decent, theres very few pilots that will spend time looking for the reason. Its a bit like the fuel gauge; I dont believe if it says full, but I do if it says empty.
Just react, get out of the way, then sort out the reason. Its why acting in accordance with an RA is legal without telling ATC (until afterwards) if theres not time, even in controlled airspace.

@PeterCordes, it's due to false positives.  At typical cruising altitudes and speeds, minimum lateral separation is about 38 seconds.


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Why does the INIT page on the A320 MCDU have "FLT NBR" instead of "CALL SIGN"?
My understanding is that the call sign is entered for FLT NBR on the INIT page (e.g. EZY8418), not the flight number (U28418). I have heard of reports of pilots mistakenly entering the flight number instead of the call sign, meaning that incorrect data is transmitted via ADSB.
Why would Airbus use such a misleading title?

“EZY8418” and “U28418” are both flight number, just using the ICAO and IATA operator prefix respectively. But the call sign indeed may not match the flight number these days.

The underlying info is covered really nicely in this article. Long story short, flight number was used for a long time until there were so many flights the repeated numbers and similar names became too lexicographically similar over the radio to the point of confusion. Governing bodies eventually switched to an alphanumeric call sign to avoid these issues.

To reduce the likelihood of call sign confusion resulting from call
sign similarity, airlines and ANSPs turned to alphanumeric call signs.
Alphanumeric call signs replace the formerly purely numeric call signs
and are assigned to flights to in such a way as to maximize the
differences between call signs and minimize the likelihood of
confusion in a method called deconfliction.

Its simply a hold over of older verbiage in the display.
We also now live in a world where flight number may not be truly unique.


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Can air traffic control see control panel selections (I.e. FCU on A320 or MCP on B737)?
I have flown with a few captains that have told me that air traffic control can see the settings we set on the FCU (or MCP on B737):
Is this actually correct? I'm aware that ATC can see what speed, heading, altitude, V/S, etc. we're actually flying, but can they see what we have selected on the control panel?
I'm not sure if it makes a difference, but I am in Europe.

ATC  can see Selected altitude, true airspeed, Mach, indicated airspeed, track angle rate, true track angle etc on aircraft lable if the automation system (radrar) is Mose S enabled. In India, we can see these data and it seems Europe too can see Selected altitude and provide alerts when there is a mismatch between cockpit selected altitude and cleared altitude

Apparently they can if the ATC facility supports Mode-S EHS transponders. According to the linked Skybrary article

Aircraft compliant with Mode S EHS provide ELS functionality features (see above) plus some or all of the following downlinked aircraft parameters (DAPs):

Selected Altitude - the flight level which is manually entered in the FMS by the pilot[...]

Emphasis on the FMS is mine as the altitude selected on the MCP does not necessarily match the altitude selected on the FMS. For the sake of completeness, Mode-S EHS also provides:

Roll Angle, True Track Angle and Track Angle Rate
Ground Speed
Magnetic Heading
Indicated airspeed (IAS) and Mach-number
Vertical rate (barometric rate of climb / descent)
TCAS downlinked resolution advisories

But for more details on each of the bullets I would strongly suggest that you read the article. You will see for example that the selected altitude or the roll angle will most probably be used by the algorithms backing the system and not by the controllers themselves.
A small note on this one though:

I'm aware that ATC can see what speed, heading, altitude, V/S

All the above (with the exception of altitude) are calculated with the available ground equipment and are not based on the aircraft readings. From the altitude, the radar system has to deduce the vertical speed. And there is some terrible rounding taking place as the accuracy of Mode-C is 100 feet. So if you have a radar scanning at 5 seconds period, an aircraft with a low RoCD (less than 1200 fpm in that case*) might appear as maintaining the altitude between 2 subsequent scans while it's actually descending. All I'm saying is that (with the exception of the altitude) the rest are as good as the ground system processing them. Mode-S is meant to change that but this is still work in progress as far as I know.

*That might need a bit of an elaboration:
60 seconds / 5 seconds = 12 scans per minute times 100 equals 1200fpm the vertical "resolution" of the radar. What I am not taking into consideration is the Mode-C rounding and the case of having double (or triple) radar coverage and in that case the radars most probably will illuminate the track at a different time. As I am not a radar expert though, I can't provide any more details and 1200 might be an exaggeration while 600 might be a more certain value. I know though for sure from people who worked on these systems, that back then when they designing the system, V/S computation from Mode-C was a challenging aspect.

Most air transports built within the last 20 years give or take are EHS-capable. Older aircraft may have EHS-capable Mode S transponders (due to common spare parts pool) but will be EHS-capable only if they've had the necessary wiring changes to get the parameters to the transponder. Many have been retrofitted, but there may be some that have not. I believe there are places that require EHS to operate in their airspace.


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What is this four-engine propeller airplane?

I came across this print at a bargain store a couple weeks ago. At first I thought it might be a variant of a Lockheed L-188 Electra; then I noticed that it has at least 6 engines, as the print extends around the edge of the canvas and you can see a couple of propeller tips there. I haven't had any luck finding a 6- or 8-engine aircraft that matches it.

Edit: after looking at it a little closer, I think the edge might be a printing effect, an imprecise mirroring around the edge line (for example, some of the splotches look very similar to those in the front). So maybe it is only four engines?

The extra engines are photoshopped mirroring to disguise the frame-less edge -- the effect is jarring when used with what is already a simple symmetric image.

@amI: You should make this an answer.

I believe it's an Ilyushin Il-18. Pic from here:

That's it!  Well done.  I thought it was a Brit where the artist had taken some liberties with his reproduction.


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What is the stability of a 777-like plane if the engines flameout and the autopilot disconnects and the pilots don't make any inputs?
According to Wikipedia:

If no control inputs were made following flameout and the disengagement of autopilot, the aircraft would likely have entered a spiral dive and entered the ocean within 20 nmi (37 km; 23 mi) of the flameout and disengagement of autopilot.

Does this mean the autopilot automatically disengages when both engines experience flameouts? If so, how likely is it it would enter a spiral dive within 37km of the flameout?
Are there any other control inputs the pilot could have made before flameout that would cause the plane to glide until it impacted the water?

The pitch stability will be positive, otherways the 777 could not be certified.

Assuming CG is within limits, the plane is straight, level, and both engines flame out simultaneously, lacking any control inputs, the plane will dive until it reaches its elevator trim speed, then glide at that speed until it impacts.  As the directional and roll stability would be fairly good with the 777, without variable cross wind effects or any other yawing tendencies, it would probably be a straight in crash.
This scenario could be tested under controlled conditions.

"Lacking any control inputs" would possibly result in a Phugoid oscillation.

Yes, in the glide forward CG pulls nose down if too slow, elevator trim raises nose if too fast.  "Phugoid" only happens when trim is excessive and plane cannot stabilize.  Also caused by pilot inputs out of sync to what the plane is doing (More common on larger planes).  Elevator trim for cruise, left untouched (not enough) would more likely result in a fast dive with little oscillation (and lots of pieces).

@RobertDiGiovanni: A phygoid is very weakly damped, so if the trim point is at a markedly different speed than the starting point of the maneuver, a phygoid motion is a certain outcome. It will die down over time but certainly go through several cycles. Also, I expect the roll instability to appear sooner rather than later, so if there is more than a few 1000 feet of altitude to burn, the aircraft will start to enter a spiral dive.

@PeterKämpf For a double flameout in cruise, though, I‘d expect a trimmed condition to start with. 777 is roll direct, so not much roll damping from control laws I guess. I‘d also be very surprised if it made it straight in without any control inputs; spiral (even a slow entry one) is much more likely.

@CptReynolds: Roll damping is provided by the wings. Roll stability, less so. And I agree with you that 1) a trimmed starting position is most likely, and then no phygoid would be noticeable and 2) a slowly developing spiral is most likely.

@PeterKämpf Thank you. It’s been a while since I actively worked in flight controls, but isn’t roll damping also provided in the control laws e.g. for roll rate command laws as in the FBW Airbusses and 787?

Although the proof would be in the testing (Yes! It is so important to throw those balsa gliders at their proper trim speeds! (Or they do phugoid before settling)), please note that the trim speed IS NOT markedly different from the starting point of the maneuver and is in fact properly trimmed for cruise (the plane will attempt to maintain this speed by diving).  As far as spiralling, that may start, but our human (distance in miles, height in feet perception) skews the scenario.  It will impact in seconds, with maybe a few degrees of turn.  But I yield to actual flight test data.

@CptReynolds: Yes, the control laws add artificial roll damping and add one more by bringing the wing back to the old roll angle (minus gyro drift) after a disturbance.

@PeterKämpf Ah, see, too long ago for me. I would have thought it’s rate control/attitude hold for the Airbusses (which could leave them banked post-disturbance, if said disturbance is severe enough to not be completely controlled to zero at zero-rate demand). I‘ll re-read the beloved Brockhaus.


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Is the destination of a commercial flight important for the pilot?
As has been recently advertised (in here, here, here also), a recent British Airways flight from London City Airport destined for Düsseldorf mistakenly flew its passengers to Edinburgh instead.
I'm a layman in terms of aviation, but I (and I'm sure many more do) do see aviation as a strictly regulated area full of double-checks, double instruments (even 2 pilots!) to make sure an airplane is doing exactly what it's supposed to be doing.
Perhaps mine is a rather car-centric view, but I can't imagine starting the engine of the car without knowing clearly where I'm heading, which landmarks, roads (and so on) I'm supposed to pass by during the route.
TL;DR:
Must (or should) both pilots know where he airplane they are flying is going to? Or do they simply follow an approved flight path?

In reality that flight was not operated by BA, rather it was a flight contracted from BA to a 3rd party. The pilots received information to go to Edinburgh and that's what they executed: this probably because of a mistake that happened in the chain of control. The only thing I am wondering is what they said on the PA

@Afe A simple call from the pilots: "Our estimated flight to Edinburgh is 1-hour and weather is fine there" should suffice!

I was on a flight once where the pilot said, "Just checking everybody on the plane either wants to or has to go to [small backwater town]."

@gmauch everyone was already sleeping before T/O?

Flying surely is nice regardless where but much less so about the landing. Somewhat strange question. Voting to close.

The suggested "duplicate" question, discussing Gross Navigation Error checks, is about a separate topic. That's about getting off of the course you intended to fly. That isn't what happened in the referenced incident; the crew landed at the airport they intended to land at -- no navigation error occurred.  The error that did occur was elsewhere in the process, & no check for navigation errors would have caught it. Close the question as "goofy" if you want, but it isn't a dupe.

If you're a limo driver and you're told to go pick up some kids and take them to prom at a local high school, you wouldn't have any reason to question whether your dispatcher might've told you the wrong high school...

The pilots in this case did know where they were going: Edinburgh.
BA said a paperwork error was to blame, with the pilot following orders from Germany, where WDL’s head office had filed the incorrect flight plan.  The pilots flew according to the flight plan they were given, and ATC happily routed their plane along that flight plan as well.  The pilots had no reason to suspect the flight plan was incorrect, and safety was never compromised.
The dispatchers involved could use improvement, though.

Would a reference improve this answer?

added reference

So the pilots aren't always aware of what the passengers' tickets say?  Never really thought there'd be a knowledge gap there.

@BruceWayne I don't remember ever being on a flight where the crew didn't say "Welcome aboard... service to "... although whether anyone would say anything if they said the wrong destination I don't know.

@TemporalWolf - I'm just thinking in this specific case, I wonder if the pilots said "Welcome aboard your flight to Edinburgh.", and ...nobody said "Uhhh, what?" before takeoff.

@BruceWayne I’ve actually had the pilot say the wrong destination in their announcement before, they came back on the TA 5 minutes later to apologise and said the correct destination, presumably because someone pointed it out to them. Maybe on this relatively short flight no one was able to point it out straight away and it was easier to carry on to Edinburgh then get a new flight plan once back on the ground.

Various articles indicate passengers realized something was wrong when the pilot made the pre-arrival announcement. No mention of a pre- or post-departure announcement, so either one wasn't made or nobody noticed the mismatch.

Forget needing announcements. I'm genuinely surprised none of the passengers noticed they never crossed water (the English Channel), or didn't distinguish the geography (Belgium vs Scotland), or didn't use their GPS once. Until they figured it out on the pre-landing announcement.

@smci Some did notice, but too late, perhaps. Check the first link on my question: ..."she realized something wasn't right when she saw mountains outside the plane, instead of the "usual German industrial landscape."
"When we started descending and I saw some taller hills/mountains, I did think that this isn't how Eastern Netherlands/Western Germany should look like but I assumed we took some small detour," she said. "Then my colleague sitting across the aisle from me told me to check Google Maps -- and it showed us being around Carlisle."

@gmauch: I had already read it before I posted, that's exactly what I was referencing. Ok so "on descent or pre-landing", nearly same thing. I'm stunned noone noticed the absence of English Channel early on. The point is this "Duh!" moment came hours too late to be useful.

For commercial flying, yes both pilots should know where they are going and the general direction/course they are flying but they do in-fact follow an approved path and are subject to ATC vectors. The reality is that pilots who fly short haul may fly numerous flights in a day to various destinations with limited time in between flights to regroup. Flight plans, load sheets and planning may very well be done by a dispatcher and simply handed to the crew when they arrive at the aircraft. It is possible to get confused, typically there is some navigational programing or FMS input that would force the pilot to review the path. However confusion like this has occurred before as was the case with Varig Flight 254 which simply entered the wrong heading into the autopilot and ended up lost and low on fuel.
In the case of general aviation this may be less of the case. While generally you take a small plane up to go from one place to another there are those that pull the old cub out on the weekend and just go fly around the area to do some sightseeing. This is more just for the joy of flying...


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What materials are used in a Rolls-Royce RB211-524 Engine?
I am trying to know what materials (Specifically metals) are used in making a Rolls-Royce RB211-524 Engine. I already found what is used in engines in general, but is it the same for all? And if so, what could be different in this specific engine?
Thanks!

There is some variation. The RB211 was to be one of the first engines to use a fan stage made in carbon fiber (Hyfil), but that plan was changed in a late stadium and the fan stage was done in titanium instead.  Conways did use Hyfil, IIRC.
Otherwise it's mostly steel and aluminium, with some titanium. Copper for the wiring etc.


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How to correctly interpret the taxi procedures in ICAO Doc 4444?
I have a question to ICAO Doc4444, Chapter 12.3.4.7, Taxi procedures for departure, c).
My question is how to interpret the following, knowing that words in parentheses are mandatory, and words in square parentheses indicate optional additional words.

TAXI TO HOLDING POINT [number] [RUNWAY (number)]
  [HOLD SHORT OF RUNWAY (number) (or CROSS
  RUNWAY (number))] [TIME (time)];

I assume that this can be exploded to 12 variants (not including TIME at the end of each variant which would give another 12 variants).

TAXI TO HOLDING POINT
TAXI TO HOLDING POINT A1
TAXI TO HOLDING POINT A1 RUNWAY 09
TAXI TO HOLDING POINT RUNWAY 09
TAXI TO HOLDING POINT HOLD SHORT OF RUNWAY 30
TAXI TO HOLDING POINT CROSS RUNWAY 30
TAXI TO HOLDING POINT A1 HOLD SHORT OF RUNWAY 30
TAXI TO HOLDING POINT A1 CROSS RUNWAY 30
TAXI TO HOLDING POINT RUNWAY 09 HOLD SHORT OF RUNWAY 30
TAXI TO HOLDING POINT RUNWAY 09 CROSS RUNWAY 30
TAXI TO HOLDING POINT A1 RUNWAY 09 HOLD SHORT OF RUNWAY 30
TAXI TO HOLDING POINT A1 RUNWAY 09 CROSS RUNWAY 30

My questions are:

Is my number of variants correct?
All variant are instructions to taxi to a holding point, in case 1, holding point is obvious to the pilot due to runway in use? Correct?
In all cases, the instruction gives clearance to taxi to a holding point without follow up by any other clearances, but for
case 5, 7, 9, 11 the pilot must wait at holding point short of runway 30 (for stopbar light etc. to be opened). Correct?

Before I answer, is it correct to presume that on your fictional airport:  1.)  To get to RW 09 you will always be required to cross RW 30?  2.)  To get to RW 30 you will always be required to cross RW 09?  3.)  You will never taxi to the reciprocal runways, i.e. RW 27 or 12?

P.S.  It might be helpful if you could provide a little more context in the form of a quote or link.  I tried looking for ICAO Doc 4444 for reference and it looks like I would have to buy a copy for $390!

Hi. Yes, this is a fictional airport and yes, in my example I will always be required to cross RW 30 to get to RW09. I don't want to taxi to RW27 or 12 in my case. Just want to know if I this is the correct way to interpret this taxi procedure. Regarding the document, one must by a copy so I cannot provide a link, sorry.

Below are my answers to your specific numbered questions.

I believe there are 4 variations of taxi clearance you might expect to receive in your scenario:
a.  Taxi to RW 09
b.  Taxi to RW 09, hold short RW 30
c.  Taxi to RW 30
d.  Taxi to RW 30, hold short RW 09
This is incorrect.  You will never receive taxi clearance to an undefined “holding point”.
Basically correct, but there are no traffic lights or stop bars on any taxiways I have ever seen.

Regarding #1, those are the four possible combinations of a basic clearance, i.e. you are either allowed to cross the off-duty runway or you are not, and there are but two runways.  However, that doesn't mean that you might not hear variations to this.  While pilots and controllers strive for standardization there is some variety.  For example, while not required to do so, it isn't unheard of for a controller to say "taxi to RW 09, cleared to cross RW 30."
To provide some additional context, clearance to taxi to an active runway clears the pilot to cross any off-duty runways that may intersect the taxiway.  If the pilot is cleared to taxi but told to hold short of an intersecting runway, further clearance to cross and continue taxiing will be made on the radio.
The actual words “holding point” (in my experience) are never used.  Clearance to taxi to a runway presumes the pilot will taxi right up to the painted hold-short line and stop there before switching frequencies and requesting takeoff clearance from the tower controller.  The tower controller will then clear the pilot to either take off, line up and wait, or continue holding short.
At most airfields serving general aviation aircraft there is widened section of the taxiway in the hold short area for performing engine run-ups and final takeoff checks.  However, the ground controller will never clear you to this “holding point”, but will instead clear you to the runway in use.  It is up to the pilot to use this area as needed, and then taxi up to the hold short and switch to tower frequency when ready for take off.

Hi. Thanks a lot for your complete reply.

Taxiway G @ BIRK has traffic lights half-way

Also at BIRK you first get a clearance to an engine run-up area and then from there following a new taxi clearance to the RWY once the run-up is complete.

Some airports have multiple holding points (US: intersections) along a runway, so they need to specify which one unless they want you to go to the end (the default).  Likewise, they need to specify the runway number unless the ATIS lists only one as active or the choice is blindingly obvious from the winds.

"there are no traffic lights or stop bars on any taxiways I have ever seen" Then you clearly haven't been to very many airports

Clearly I haven't compared to you.  How do you taxi in a glider anyway?


text:
Does anyone know free DIY ultralight plans?
Is anybody aware of  open source plans for an ultralight that can be made out of standarized aluminium/wooden parts without welding?

Most designers of successful ultralight designs (rightfully) believe they should be paid for their efforts.  If nothing else, this provides a financial buffer against potential legal actions.
That said, there is at least one open source ultralight glider family: the Goat.  It meets your design criteria in general, in that it's designed not to require specialized parts or materials.
There are cases of builders adding an engine to versions of the Goat or to Goat-derived designs -- the Bloop is one such -- but I'm not aware if the plans for those modification are freely avilable.

The problem with the Goat is if you want to motorize it you have to increase the GW by at least 50lbs and will have to beef up the structure.

Might be why there aren't many motorized Goats or Bloops around.  You have to know what you're doing to build one, never mind motorize it.

I have the plans and it is a pretty simple design with easy to make parts; but there are a lot of them.  There is a guy in NY state who has a motorized ultralight based on the Goat, who sells kits, but it's significantly redesigned and heavier, and the vids of it flying with fairly small motor aren't very impressive.  The Mitchell Wing can be a very good choice for a soarable ultralight. Simple wood structure, twitchy but manageable in pitch and safe to fly if you learn properly, ideal for electric power as an ultralight motorglider.  Problem is trying to find plans.


text:
Is there a good online source on rotating detonating pulse engine?
I can't find a clear presentation that would explain the workings of a rotating pulse detonation engine graphically, only useless (to a layman) cfd simulations of the combustion chamber. Please share if you know a good online source to learn about this design.

The idea seems fairly simple. Here's a picture from one article and an explanation from another.

From Ars Technica

Micro-injectors squirt a pressurized mixture of fuel and air into a combustion chamber the shape of a long ring (annular cylinder). Then, this gas mixture explodes, with the explosion spinning around the circumference at supersonic speeds. The high pressure produced by the detonation forces exhaust gases upwards (in this diagram) and out of the chamber, where they expand through a nozzle and produce thrust that can be used to push a turbine or aircraft.
Unlike a PDE, which requires repeatedly initiating detonations, an RDE only requires one detonation that spins around and around the chamber, continuously producing thrust.

So there's no conventional turbomachinery. The detonation wave moves in a circle round the chamber, jumping from injector to injector. The only moving parts are the fuel injector pumps. I'm not sure how you throttle it, though.

The injector pumps wouldn't be that far from conventional turbomachinery. Keep in mind they are injecting fuel-air mixture, not just fuel,


text:
Could a jet engine be made from cast aluminium?
What materials are used to make the high temperature-high pressure bearing parts in a jet engine? I'm asking about the combustion chamber, combustors, nozzle and diffuzers (as in ramjets) rather than turbomachinery. Could cast aluminium or cast steel do the job for a low thrust small model engine?

There's this question ( https://aviation.stackexchange.com/questions/25645/what-material-is-used-to-make-the-hot-sections-of-jet-engines). I would think it would have to depend on the temperatures you will have to withstand. There have been things like car motors, or portions made out of cast aluminum or cast steel, but those are not really seeing the temps of a jet engine.

you can, by lowering the operating temperature of the hot section, but that will make the engine very inefficient. the history of gas turbine has been pushing the temperature higher by advancing material and cooling technology but now you are trying to do the reverse.

If you're looking to build your own small jet engine, there are lots of links on how to build them using an automotive turbocharger as the core.

Short answer: No.  Unfortunately the yield strength of typical aluminum alloys drops off after about 300° F and long term exposure to high temperatures dramatically affects their creep and fatigue lives.  Typical aviation gas turbine hot sections run between 3000°-3800° F and rotate at angular speeds as high as 40,000 RPM, making Aluminum totally unsuitable for this role.  Most hot section components eg cumbustors, stator buckets, rotor buckets, etc.  are manufactured from exotic nickel-cobalt superalloys and employ novel single crystal casting techniques as well as passages to bleed compressor air as a boundary layer over them to protect them from that kind of intense heat.

Aluminum has a melting point of 660 C.
Really depends on what "low thrust small model engine" refers to. But if it burns kerosene or jet fuel, probably not for the combustion chamber.


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Could injectors be used as an alternative to turbopumps in jet engines?
Could injectors be used instead of turbopumps in aeroengines? Are there jet engimes that already use them?
https://en.m.wikipedia.org/wiki/Injector

I assume you are talking about fuel. most engines I'm aware of do not use turbopumps for fuel, but rather positive displacement pumps like a gear pump.

An injector and a pump are not interchangeable. The pump moves the fuel, the injector is the point at which the fuel enters the combustion chamber. To my knowledge, turbopumps are used on rocket engines.


text:
How could I measure frontal area based on a technical drawing?
I want to compare frontal surface areas of two airplanes, and I have their digital technical drawings and wingspan data. What program could I use to obtain the surface area using this data? Esentially a program that would let me put the drawing in a rectangle and measure the ratio of uncovered area of the rectangle to area covered by the drawing would be perfect.

Could you use a CAD program? Like FreeCAD.

If you had mentioned which type of program you prefer in terms of being freely available or complexity, it would be quite helpful. I know that you can do such thing very easily in CATIA (generative shape design workbench). I guess that all CAD software are able to do this operations.

In the good old days we used a planimeter. Can you get your hands on one of those?

Sounds like a question for https://softwarerecs.stackexchange.com/


text:
Did any VTOL aircraft implement an airfoil with engine-blown exhaust?
Did any VTOL aircraft implement an airfoil with engine-blown exhaust?

https://en.wikipedia.org/wiki/Coand%C4%83_effect#Applications

STOL but gogo d exemple of what a blown wing looks like https://fr.wikipedia.org/wiki/Breguet_941#/media/File:Breguet_941S-3v.jpg

Not possible engine blow to the air foil will make VTOL (vertical Take Off Landing). It will just make STOL (Short Take Off Landing), there is Antonov An-74 (video), and Boeing YC-14 (video).
These two airplanes are utilizing Bernouli's principle to create very fast wind blow (velocity 1, v1) above the wing, compare to the wind velocity below the wing (v2). Due to the different airspeed, than the airplane take off shorter than common airplane. From P1+0.5rhov1^2+rhogh=P2+0.5rhov2^2+rhogh, the rho is the air density (in kg/m cubic). The rhogh will cancel one each other as they has the same air and the altitude consider the same. Hence you then can calculate the v1 and v2, and then the take off distance.
VTOL is a lifting/flying process without any horizontal moving. See here some of VTOL and STOL: VTOL & STOVL Aircraft Compilation (video).
So, by forcing the low pressure above the airfoil, it will just create almost VTOL. But almost VTOL is still STOL. Here is a detailed video on VTOL: The Real Life Sci-Fi of Vertical Take-Off Planes. But not as you think.

Near as I know every attempt to do so has never worked out.   It’s a very enticing design on paper but has always been a disappointment in real life. One of the best examples was from the 1970s. Designed as a supersonic replacements for the AV-8A Harrier, the Rockwell XFV-12A attempted to use this scheme for a vertical lift system.   Unfortunately it never met the design metrics, and the program was ultimately canceled.

Source: boxartden.com

this concept is called a "blown wing" and when the airflow is concentrated over the flaps it is called "blown flaps". lots of stuff has been written about them but they always work better on paper than in real life.

There are a few drones that use a non-rotating Coanda surface (blown by a ducted fan or centrifugal fan) for lift, with moving flaps for two-axis control (throttle is used to control altitude).  This one is called Aesir, and there are a number of amateur-built copies, but I'm sure I've seen others with a different overall shape.
The advantage of this over a single-fan thrust-hover is simplified controls, no torque counteraction required, and considerably reduced power requirements.  The disadantage is the relative fragility of the large, hollow Coanda lifting surface.


text:
Why is "proverse" yaw better than no yaw?
Why is the tendency to yaw in the direction of the roll more desirable (since it's the "proverse" yaw) than the lack of yaw in response to roll? Is it even possible to have no yawing tendency at all during rolls in a plane?

I would add that it depends on what you mean by "proverse yaw", and that vertical fin size is key.
Proverse implies "too much" yaw in excess of the appropriate yaw rate for the angle of bank, which would be a skidding turn. That's bad.   If proverse just means an into-turn yaw rate that is appropriate for the bank, that's good.
Airplanes with spoilers-only for roll control suffer can suffer from "bad" proverse yaw in that sense, because the drag of the down-wing spoiler, with nothing sticking up on the opposite wing to offset it, pulls the nose into the turn too much, requiring out of turn rudder to keep coordinated flight (I've heard that the spoilers-only Mitsubishi MU-2 is like that).  But almost all airplanes have the opposite, adverse yaw, to one degree or another.
Anyway, what we want is a natural yaw rate that is appropriate to the bank angle so the tail naturally wants to stay lined up behind the nose in the air stream during the turn. Asymmetric drag effects of flight controls aside, the yaw rate you get in concert with bank to get a turn is a function of the vertical fin (and the rudder, to the extent that it's providing a passive fin-like effect, like with a hydraulic rudder, or a mechanical rudder with centering springs).
It's basically a big weathervane.  If you had no fin/rudder, no weathervaning effect at all, you'd just skew sideways when you bank (even ignoring the dynamic yawing effects of the rolling action or asymmetric aileron drag, which make the slipping worse).
You would think it's desirable to have a fin that can make the tail follow the nose immediately and precisely, when side slip occurs as a result of banking, so the bigger the vertical fin, the better.  But there's a problem with that.  To exploit the self-righting effect of dihedral, to give wings-level stability in roll, you need to have a little bit of side slip occur when the wings are banked, to create the self-righting lift differential.
So you need a weathervane, but not too big a weathervane.  The sizing of the fin itself is a balancing act over the need for strong into-turn yaw, and for roll stability due to dihedral effect.  If the fin is too big, into-turn yaw is too strong and immediate; the airplane will immediately yaw (weathervane) into the turn with little or no initial side slip, and it will want to enter a spiral dive any time it's banked.  Vertical fin too small, and the weathervaning effect is too weak and the airplane hunts and slithers around in yaw too much.
There is a sweet spot for fin sizing that gives good "proverse" yaw to give a naturally coordinated turn that only needs bits of rudder to counteract adverse yaw while the ailerons are displaced, while still allowing just enough initial side slip to allow dihedral effect to apply its self-righting tendency when the airplane is banked slightly by a bump.

Very interesting answer, shows your experience with large swept wing airliners, and why so many people consider the slip after the roll so important in understanding the mechanism of the turn.  With swept wings (learned from Jan and Peter) the inside wing is now "straighter" into the relative wing.  That (added drag), along with the "weathervane" gives the "natural" coordinated turn.  Aileron turn, helped as needed with rudder.  Funny, the high straight wing 172 is a rudder turn helped by ailerons. It's all good coordinated.  Thanks.

Well this applies to all airplanes but with sweep it gets way more complicated because the sweep adds yaw stability allowing a bit less fin area, but also creates really strong roll/yaw coupling leading to dutch roll problems and the need for yaw damper systems.  Jets require very small amounts of rudder to correct for adverse yaw and the yaw damper system takes care of it.  You never touch the rudder pedals on a jet once airborne unless an engine fails.  I don't really see airplanes as turning with rudder because the turn is due to bank and the rudder is just to keep the tail  behind the nose

Yes, as usual what adds to stability going forward can add to instability in the slip.  Since many people move from trainer type aircraft to airliners, does the training highlight less rudder? It could for a number of reasons for that type of plane.  For something really bottom heavy (like a plane with floats or huge tires) a little more rudder with dihedral might help.

You quickly learn on your first type course to put your feet on the floor during the climb out once you no longer have to be ready for an engine cut.  I had a bad habit from years of gliders, taildraggers etc, to leave them on the pedals, unconsciously giving little squeezes of rudder with aileron, and had to consciously stop myself and let the yaw damper do its thing.

A two part answer here.

Yawing in the direction of the roll in a turn is known as a "coordinated turn".  How to do it depends a lot on the design of your plane.  In a high wing dihedral (trainer), with slight aileron deflection for a left roll, the dihedral and pendulum will neutralize the roll and the downward right aileron may actually start to skid the plane to the right!  This is why you "center the ball" with left rudder and push the leading right wing into the relative wind.  This overcomes the planes stability and gets it to roll and turn to the left.

Very important is that with any movement in an aerogravimetric environment (earth and sky)
any acceleration reaches "steady state" when DRAG forces match accelerating force.  The pilot can also "ease up" on the input to lower the rate of change.

Yawing while rolling.  Let's sort out effect of CG on aircraft during roll first.  A forward set CG requires rudder and elevator inputs during turn to keep the nose level to the horizon.  Notice, during the roll, they swap roles as the plane rolls through 90, 180, 270, 360 (0).  Moving CG back (to a nuetral static stability) eliminates this effect.

What we are left with is the yawing tendency due to differences in AOA of upward and downward rolling wings, cancelled by aileron deflection as Peter pointed out.  Depending on the design of the plane, it might not be much.

Is it even possible to have no yawing tendency at all during rolls in a plane?

No.
Once the aircraft is in a steady roll, the roll damping will fully compensate for the differential lift caused by aileron deflection. Now the rolling motion will increase the local angle of attack on the downward moving wing the farther you are away from the center, and decrease it on the upward moving wing. Since lift is perpendicular to local airflow, the air force vector on the downward moving wing is pointing slightly forward, and that on the upward moving wing slightly backwards. Please note that the total lift produced by each wing is roughly the same! The rolling moment of both is exactly zero once the roll rate is steady. However, the aileron deflection will decrease lift on the downward moving wing such that it just compensates for the increased angle of attack due to the rolling motion, and the same holds for the upward moving wing with its trailing-edge-down aileron deflection.
What is different is not the magnitude, but the direction of lift on both wings. This pulls the downward-moving wing forward and the upward moving wing back, causing a negative yawing moment in a positive rolling motion. That is why you will always have a yawing moment in a roll.

I've got 1000+ hours in an aircraft that with yaw damping off, when you throw the ailerons over full deflection the plane's nose just keeps pointing forward while the plane slides off on whichever wing is down. Neither adverse nor proverse. E-2C Hawkeye.

@qqjkztd: Please read the answer carefully. The rolling motion changes local AoA regardless of initial AoA. The resulting yawing moment will not even become proverse in inverted flight, or a vertical dive for that matter.

@MikeY: With a big tail in the prop blast you simply never noticed the yaw, because it is so small. Turn off the props and repeat. The yaw is there, only masked by the increased dynamic pressure on the verticals.

@Mike Y that top dish reminded me of the circular wings from the 1920s.  No wonder you can roll it smooth, it's a biplane!

@Robert, it's what you get when you let a bunch of radar engineers design an aeroplane. Oh, you mean it has to fly too? Small rudders but lots of throw = very odd lateral directional stability behavior. Like a horse that knows the barn is behind him.

Might be why they put one big rudder on the B-24 and made it the Privateer.  The interesting tail might be there to help prevent a radar return from being bounced back into the dish.

Turns with balanced flight at less than 90 degrees angle of bank (but more than zero) are actually rotations about two axes at once in steady state. It is a continuous combination of pitch and yaw. For it to be a level turn, the yaw has to be in the same direction as the angle of bank. With proverse yaw, on rolling into a turn the yaw is in the right direction.
For an aileron roll, you can get zero yaw on rolling with a little use of rudder, or if the aircraft uses a mix of spoilers and ailerons.
In the landing pattern, a strong proverse yaw while low and slow could contribute to an approach turn stall, same as if you tried to “rudder” the turn. Not a good outcome.


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What happens when you apply full cyclic + collective?
Can a helicopter apply full cyclic and then full collective at the same time? Would'nt the blades on one side (opposite of the intended tilt direction) have already maxed their aoa, resulting in a net dissapearence of dissymmetry of lift, with the effect equal to just applying full collective? Or does full collective + cyclic result in even greater aoa (the inputs add up)?

Full cyclic in which direction? the side of retracting blade?

Multiple questions here. Assuming by full cyclic you mean pushing the cyclic to a mechanical stop while pulling full collective to the mechanical stop, the answer is yes, at least for the Bell 206, a very common turbine helicopter in use worldwide. There are no restrictions with regards to full flight control inputs. At stabilized hover on a no wind day there is no appreciable dissymmetry of lift as aerodynamically, there is no advancing or retreating blade. If you push the cyclic straight and full forward, mechanically tilting the rotor disc, the instant you move forward you now have dissymmetry of lift between the advancing and retreating blades

Yes it is possible to apply both max collective and max cyclic.
The three actuators that move the swashplate obviously have limited range of motion - what happens if some of the range is consumed by cyclic stick deflection, and collective action will consume more than the remaining actuator range. Is how I now understand your question.
In larger helicopters, the control mixer is designed with the following priority:

Collective: the power handle.
Pedals: the anti-spin device, also for when you need to apply more power.
Cyclic: for reaching your destination.

Collective and pedals have major safety related functions.
How the priority is implemented depends on the size of the helicopter, larger ones have irreversible boost actuators between control and swashplate, and are designed such that at the cyclic travel stops move as a function of collective/pedals. The stick travel limits - with the stick inside the limits there is no effect with boost on, with the cyclic fully extended it will come back a bit at full collective.
Smaller heli's without irreversible boost will act like a lever, with collective or pedal at one end and the cyclic at the other. The lever arm will favour the collective/pedal end.


text:
Do ramjets use variable geometry supersonic inlets?
Ramjets are very often portrayed as super simple, I've read on few occasions that the only moving parts in a ramjet are the fuel pumps. However, turbojets operating at similiar speeds (mach 2 - mach 3) use very complicated supersonic intakes with variable vanes to maximize the number of shockwave reflections and their angle and to adjust for the refraction of the flow on these shockwaves. Why is that the ramjets do not require such advanced measures to maximize pressure recovery?

Why is that the ramjets do not require [variable geometry] to maximize pressure recovery?

Turbofans, in aircraft that have variable geometry intakes, such as in an F-111 or F15, operate from standstill, at the start of the runway, to about Mach 2.5. That means, they go from having no shock, to having quite a large (strong) shock. The intake needs to provide the about the same mass flow to the engine, regardless of the forward speed of the aircraft. And, at high speed, it doesn’t want to spill excess air, because that creates spillage drag, especially when the intake has sharp lips, need to reduce form drag at high speed. But, at low forward speed, it needs a bigger intake area, as the forward motion of the aircraft isn’t providing a lot of air. So, the intake has conflicting constraints. It needs a small area at high speed, but a large area at low speed. It also needs to form an oblique shock at high speed, as an oblique shock has less total pressure loss than a normal shock wave. The oblique shock wave needs to be kept out of the intake, hence typically a spike protrudes forward of the intake. The spike moves forward as the aircraft speed increases, and the geometry is made such that this spike reduces the intake area as it translates forward.
Now, consider a RAM jet. It typically operates from Mach 3 to 6, say. So, it’s going from a speed at which shocks exist, to a speed at which shocks still exist. It might still be an operating range of 6-3=3 Mach, compared with 2.5-0=2.5 Mach, so not much different. But, in terms of the change in fluid behaviour of the air, 0 to Mach 2.5 is a much much larger change in behaviour than Mach 3 to 6.
For example, the angle of an oblique shock at Mach 2.5 is 23.6 degrees. Mach 3 is 19.5 degrees and Mach 6 is 9.6. (See formula here at NASA). So, between zero speed an Mach 2.5, the shock changes from not being there at all, to starting as a normal shock of 90 degrees, to having an inside angle of 23.6 degrees (a change of 90-23.6=66.4 degrees). In comparison, for the RAM jet, the shock only changes angle by 19.5-9.6=9.9 degrees. It’s pretty easy to accommodate a change in shock angle of only 9.9 degrees with the same geometry.

There were some subsonic ramjets (there were more subsonic pulsejets, but it seems there were some continuous burn subsonic ramjets too). On the other hand while M6 is the technical limit for ramjet (higher speeds require scramjet), is there even any mass produced ramjet that would exceed M4?

Hi @JanHudec.  I am not sure if there is any mass produced ramjet that exceeds M4. To be honest, the Mach numbers I used were to illustrate the relatively small change in shock angles for a speed range at high speeds, compared to a similar speed range over low speeds.

The thing is that 10° is still a pretty big deal, because the shock wave has to hit just behind the outer intake lip for good efficiency. However, most practical ramjet engines are accelerated by rocket booster (or embedded rocket) to the design cruise speed and then operate within maybe half a Mach, so the difference is much smaller still.

@JanHudec. Interesting info, thanks. If I was designing the intake, I would ensure the shock falls as you describe for the high speed condition, then at the lower speed, it will fall a further 10 deg away from the intake lip.

Well, it is just some internet research, really. Currently the only production use of ramjets is in missiles, which either launch with rocket booster, or are ramjet-rocket combination.


text:
How do I make consistent landings from the (instrument-less) backseat?
When flying the Super Decathlon from the backseat, I can see the manifold pressure (top left corner of the panel) and G-meter (top right corner), and that's it.  This (along with the fact that the guy in the front seat blocks the view of the runway centerline) makes it difficult to get good consistency in landings.
I know that in if your airspeed indicator fails for some reason you can simply keep a little throttle in, fly until ground effect, and then bleed off whatever speed remains, but that's inconsistent and ugly.  I'm talking about being able to hit any arbitrary point on the runway ten times in a row.  I would call that impossible without at least an airspeed indicator, but older, wiser pilots have been doing it for years.
How do I make consistent landings from the (instrument-less) backseat?

Since nobody else has posted an answer, I follow Dan's suggestion and copy my comment here. Unanswered questions make this site look bad.
Practice, practice, practice. It's for a reason that the pilots who fly consistently good landings are the older ones. And remember where the horizon is in reference to the airframe and how the engine sounds. At approach speed (= at high angle of attack) the aircraft will show large attitude changes for small airspeed changes. Remember how high you are at conspicuous points along the approach, don't be shy to look sideways for this. And keeping a little throttle makes better landings, so don't feel negative about this.

Pitch attitude controls airspeed in an approach. So if you don't change your attitude, the speed won't change, so you don't need to look at an ASI. Just fly the pitch on feel.
Throttle controls sink rate on an approach. Just look at the runway/targetpoint - if it's going up the windscreen then you're sinking and you need more power, if it's going down the windscreen then you're climbing and you need less power. Adjust the power to keep your target point fixed in the windscreen.
These rules apply to any airplane, not just the Decathlon.

Hi RAC, thanks for the answer!  I'd love to look at the runway/target point, but I can't see it from the backseat; there's another pilot whose head is in my way.  That's why I asked the question.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How are airliners AoA-Sensors calibrated?
Airflow around the fuselage is not free, even near the aircraft nose. I assume some correction is applied to the measured angle to get the corresponding freeflow angle.
How is the reponse curve for this correction obtained (for narrow/wide-body airliners)? There must be some other sensor to compare with. This cannot be GPS, since that would give pitch, not AoA.

Note that [https://aviation.stackexchange.com/q/63879/38541] is about testing, not getting the response curve.

During development testing the airplane has a "test boom" which in additional to pitot and static inputs has traditionally used mechanical vanes driving pots or resolvers to measure pitch and yaw in the free stream.  Nowadays they are using "smart booms" that do the same thing by measuring pressure distribution at the tip of the boom and working it out with software, but it's still just a way to measure the free stream out where it's minimally affected by the fuselage body.
From that data and from the AOA vane itself, the variations between AOA vane and true are plotted and calibration tables are loaded into the Stall Protection Computer software so the SPC can correct the AOA data internally before it is used for the various functions that need AOA information.


text:
Materials for temporary dent repair?
On a recent commercial flight (A320neo), I noticed some materials attached to the leading edge of a slat (no photo available). The attached material was not smooth, it rather looked like a small ball of steel wool. Making pilots aware of it, I was told this was a temporary repair for a dent.
I was satisfied for the moment, but rethinking the matter later, I wondered if dents can be temporarily fixed by spoiling the airflow, I would rather have expected a smooth foil or plate to restore the nominal flow. The aircraft continued its next legs without noticable delay.
How are dents in wings temporarily fixed?
Can pilots judge, if a damaged fix is within allowable limits?

I would imagine it's the ground-based mechanics, more than the pilots who determine what's necessary to get the plane in the air. Of course, it's the pilot who has the final say (legally and it's his life on the line, not the mechanic's), but again, I'd imagine they lean heavily on their mechanics. Much the same way you and I do when we take our car in for major service or minor repairs.

What usually happens is the crew doing the walkaround spots something that looks like damage, calls airline maintenance, and techs come look at it.  For a ding, they will consult the Structural Repair Manual guidance on allowable limits on dents.  Based on the SRM it may be ok to leave as is, or require repair, or require further evaluation.
If the SRM doesn't cover it, or not all the eventualities, they will call the OEM's product support organization for an assessment, repair scheme, and a temporary "fly-by" (a fix deadline) if feasible.  A really big airline may have its own internal structural engineering team with its own design approval authority up to some level of severity, in which case it's all done in-house.  Smaller airlines will call the OEM and pay the OEM for its engineering services.
In any case if it's outside the SRM limits, somebody somewhere with authority has to review the dent and decide what to do.
We don't really know what was done in the case you saw, so I'll speculate that the "ball of steel wool" you saw was actually the dent but looking like a bump due to lighting, and the area was probably roughened with Scotchbrite or something similar during the inspection, creating a steel wool look.  There could have been a Non-destructive Test done (usually dye penetrant) to look for cracks in the surface where the metal was bent, which would make the surface look different from the surrounding area with the treatment process.  Also if there were sharp defects like a gouge, there was probably some "blending" of the local material to minimize the stress riser effect of the gouge and the blending with emery cloth (sandpaper) would certainly leave a steel wool look to the area.


text:
How does engine geometry affect flight stability?
News items around the recent aviation accidents involving Boing 737 MAX 8 aircraft suggest, that bigger engine diameter demanded for a different (longer) nacelle strut for proper ground clearance which in the end had a negative effect on flight characteristics, necessitating an additional control system MCAS. This reasoning may be correct but it is not self evident IMHO.
One would think that a different engine mount point primarily shifts center of gravity, which can be statically counteracted by shifting other masses. The attack point of thrust, which also generates angular momentum if not coincident with the center of aerodynamic drag, should not be significantly affected by this geometry change.
So how does this geometry change adversely affect margins of inherent static stability and impede return from increased angle of attack to equilibrium?

The consensus here is that the area of the larger cowlings ahead of CP causes the pitch up effect aerodynamicly rather than thrust/drag imbalance.  One other factor to consider is the increased amount of thrust at low speeds a fan would provide compared to a lower bypass jet.  Even if the torque arm is shorter, the pitch up may be greater.  This is why a higher bypass fan would have better short field performance and again leads to a need for greater pitch control, and, better training for the pilots.  It may not be "just another 737" in its handling characteristics.

The problem appears to be mostly the effect of a larger cowling, moved more forward, that generates significant lift when operating at higher AOA and has an effect similar to moving the entire wing forward, or moving the center of gravity aft, partially unloading the horizontal tail in the flaps up low speed regime and making the pitch stability a bit neutralish.
Seaplanes suffer from a bit of a similar effect in yaw from adding floats. The floats add quite a lot of side area forward which reduces the fin's weathervaning tendency.  This is why you usually see additional fins added to the tails of float planes to restore the original yaw stability.
In the case of the MAX it appears the pitch destabilizing effect of moving the cowls forward really caught them by surprise because the problems were only found during flight test.  To restore the original relationship of the effective center of lift to CG would have required some pretty radical redesign, say like adding a plug to the forward fuse to move the operating CG forward, which would also require making the tail bigger... and off we go on the one-thing-effects-another design change merry-go-round without end.  What a mess.  They were really between a rock and a hard place, and an artificial stability system using the stab trim system to work around what was a fairly narrowly focused problem provided a relatively easy escape route.

You say the problems were only found during flight test. Is there some information publicly available on how these tests worked out? I ask because I expect any docs to shed some light on theoretical foundations of aerodynamic stability.

Sorry I'm not aware of any public documentation available, but Google "stability augmentation using pitch trim" or similar word strings and you might stumble onto some publicly available papers.  I understand that this type of system is nothing new for Boeing.


text:
What is this business jet?

It could be a Learjet, but I can't find a match.

interesting. I thought it might be a Dassault Falcon, but can't find a match in that range.

The tail is quite unique. I thought about Embraer Phenom, Hondajet, and Cessna Citation, but it is none of these.
I believe that your images are showing a British Aerospace 125 (aka Hawker 800/1000, or DH125 Jet Dragon). They were produced from 1977 to 2013, and most of them have been in service for the air forces of Japan, Brazil, and Saudi Arabia.

Source: wikimedia.org

Yes, that looks like one! You made my day bogl, thank you very much.

Not really, was looking through some old Miami Vice episodes and trying to figure out what plane was being used :) Thank you for taking your time and answering bogl :)

@Chris Miami Vice would have been a good piece of information to include - at least dates the craft to active service in the mid-late 1980s.


text:
What are these ramps on the back of the USS Midway?

Can anyone explain the purpose of these three ramps on either side of the stern on the aircraft carrier the USS Midway?

FWIW, they aren't used by pilots for anything...

Figured as much… thought it might be worth asking anyway. Know any other community that might have the answer?

as a guess, they might have been put in as a means to dump munitions in case of emergencies. The curved edge of the deck may have been deemed to drop things too close to the stern of the ship for safety.

Actually, that triggers a flicker of memory…. I think @jwenting might have it there.

Maybe not:  https://www.quora.com/What-is-the-name-of-the-ramp-used-to-offload-ammunitions-off-a-navy-carrier

You've never been on the slide at your local swimming pool?

Ammunition dump makes sense. They would need to get it over the netting, and also make sure it didn't get dumped in the middle where it fell on the deck below.

@DJClayworth - "in the middle" of the flight deck they'd still be clear of the fantail. But they'd land close to the rudders and screws, which could have negative consequences.

They use those ramps to chuck off the disobedient kids won’t stay quiet during the tour!

Those are EOD jettison ramps. (Explosive Ordinance Disposal) They are in many locations around the flight deck. A bomb/missile could be "cooking off", it may have been exposed to prolonged jet exhaust, fire, or any number of factors, any of which lead to the urgent need to get it overboard (jettison it) with the quickness.
Barry Healan, former ABH3 flight deck crew member aboard USS Coral Sea CV-43 and USS America CV-66

Nice, essentially the same as another answer, but citing your experience gives an indication as to why it should be considered credible.

In the event of an aircraft fire, ordnance (bombs and missiles) could be pushed over board down those ramps to prevent exploding and causing more damage.

Welcome to aviation.stackexchange, good first answer. However, could you please improve it by stating your source? E.g. a book, or article or perhaps you worked on these ships?

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.

authors: Bob Jarvis - Слава Україні


text:
Where can I find coordinates for Special Use Airspace (e.g. restricted areas) in the USA?
I am looking for exact specifications for a few prohibited and restricted areas near me so I can plot these areas in other mapping software.
I can use sectional charts to estimate, but I am curious if there is some kind of listing that contains coordinates that define these SUAs.

As Pondlife says, this answer tells you where to look: All special-use airspace is defined in JO 7400.10. Version C is current as of February 16 2021.


text:
Why is this Instrument Approach NA for certain radials, yet it depicts a feeder route for those radials?
The RNAV (GPS) 28 at Tampa Intl (KTPA) says

Procedure NA for arrivals on PIE VORTAC radials 356 CW 157

That seems to mean the approach is not available for anyone generally northeast of the PIE VORTAC. Why is there a feeder drawn on radial 085, clearly within the 356 CW 157? Moreover, the whole approach procedure is within those radials east of PIE. Does the NA comment simply mean a pilot must be coming from the west side of PIE when they request the approach?

Your confusion may stem from the way you are viewing the instruction not to use it for arrival on the radials 356 to 157. If you were arriving on the 356 Radial of the PIE VORTAC, your course/track (and/or heading in calm winds) would be 176°. If you were arriving on the 157 Radial of the PIE VORTAC, your course/track (and/or heading in calm winds) would be 337°. If you were arriving on the 85 Radial of the PIE VORTAC, your course/track (and/or heading in calm winds) would be 265°. All of these arrivals would necessitate a hard, sharp turn at a potentially steep bank angle. This will need to be avoided.
You are able to use other routes to the Initial Approach Fix other than flying first to the PIE VORTAC. Like flying directly to the RUYKI waypoint as your first point on the IAP. This is especially valid since arriving on any of the above mentioned courses/tracks will place RUYKI in easy access of the pilot. This would be much simpler than trying to make that hard, sharp turn at a steep bank angle. Only minor adjustments from the arrival course/track at an appropriate distance from RUYKI would need to be made. This has the added benefit of reducing confusion and freeing up airspace from conflicting traffic.
If you are concerned with using the STARs as feeder routes, you would use the PIE VORTAC as your transition to the IAP from all STARs with endpoints West of KTAP. You would fly directly to RUYKI from all STARs with endpoints East of the IAP. If you have to lose altitude from your descent from the STARs, you would use the Hold en lieu of a course reversal to do so.

Normally you transition from a STAR to an IAP, but your point is correct. RUYKI is an IAF and you would not typically use the PIE VOR for the transition. You could easily transition to RUYKI from LAL (east) or OCF (northeast). Also IIRC, almost all TPA STARS include radar vectors long before you reach PIE.

Thanks @Gerry. That was a late night error. I meant STARs.


text:
How close to a Restricted Area can a pilot be cleared to fly?
In my local area there is an instrument approach that skirts the edge of a restricted area by a mile or two. The TRACON for this approach is not the controlling agency for the restricted area. When the restricted area is active, the TRACON will not give clearance for the instrument approach unless the weather is VMC. Is there a requirement that ATC cannot have pilots flying too close to Prohibited/Restricted areas in IMC? Even if they're flying a published procedure?
Is this different for VFR flights?

"TRACON will not give clearance for the instrument approach unless the weather is VMC" You mean unless the weather is IMC?

Negative. They will not allow us to fly close to the restricted area unless we are VMC (they might have us cancel IFR)

ATC will separate IFR traffic from active special-use airspace according to the separation minima prescribed in the 7110.65 9–3–2:

Vertical separation: 500 feet above or below a prohibited/restricted/warning/MOA/ATCAA/ALTRV area (1000 feet above or below, if above FL290).
Radar separation: 3 miles from the periphery of special-use airspace; or for specific areas that have been designated by management as not needing 3 miles, simply "clear of" the periphery.
Non-radar separation: On airways whose protected airspace does not overlap the special-use airspace. (Remember that Federal airways extend four or more miles from each side of the centerline.)

Note that VFR traffic are explicitly mentioned as not prohibited from flying through a transitional hazard area (a term I think is used in conjunction with spacecraft launch and reentry). It is also "common knowledge" that VFR aircraft may fly through an active MOA but not active R- or P-airspace; however I can't seem to find anything that says this explicitly. R- and P- airspace is discussed at 14 CFR 91.133, which does not mention MOAs.

Without knowing the specific situation you're talking about, my guess is that when the restricted area is hot the TRACON is withholding an approach clearance using that approach for all IFR aircraft whether IMC or VMC, and only permitting VFR practice approaches to use it.


text:
If I have a VOR receiver and a GPS, which should I use to navigate VOR airways?
FAR 91.181a states that aircraft on ATS routes shall be flown along the route centerline. I would like a clarification regarding VOR airways. I have a GPS and a VOR receiver, and usually when I fly VOR airways I set both to navigate the leg I'm on. This gives me two navigation needles and two CDI sources. Usually they both line up very well, especially when close to the VORs themselves. A few times when I've gotten far from the station I've noticed a discrepancy between the the VOR nav and the GPS. I assume this is due to inherent error with the VOR as the aircraft gets farther from the station. When this occurs, which navigation source should I use?
I understand VOR airways have 8nm width to account for VOR error, but again, according to FAR 91.181, pilots must fly along the centerline (as best they can anyway). So will the GPS get me closer to the centerline, or the VOR receiver?

Is your GPS certified for IFR navigation? And, are you operating under IFR?

@RalphJ: look at the tag

I don't rely on tags to be part of the question, @PeterDuniho, both should be put in the question explicitly.

will the GPS get me closer to the centerline, or the VOR receiver?

GPS is usually more precise than VOR. Even when selective availability was used this was true, and that program hasn't been in effect since 2000.
The GPS will also have approximately the same error along the entire airway. The VOR's error will increase with distance from the VOR station.
That said, IMHO there's a difference between "which is more precise" and "which one should I use". Airspace features such as airways increase the chances of a collision by concentrating air traffic. An airway isn't as bad as an intersection or airport, of course, and flying appropriate altitudes for your direction of flight mitigates this risk even more. But I would tend to want to avoid being right on the centerline.
Many GPS receivers (and other RNAV devices) allow you to offset your course, so you can fly the airway that way, with a randomly selected offset that remains within the airway†. Or just use the VOR, and let its natural lack of precision help you to avoid being smack where all the other air traffic that's flying exactly on the centerline is.
† Strictly speaking, one should request the deviation from ATC, to avoid running afoul of FAR 91.181, which requires flying the centerline…it is unlikely they would object, but if they do, then you will find out the safe way

Interesting. Isn't flying offset from the centerline directly against FAR 91.181? Also, isn't the concept of airways all about making it easier for ATC to provide separation (as answered by this question https://aviation.stackexchange.com/questions/56614/why-do-rnav-airways-exist)?

@asb1230: 91.181: I suppose technically, you should request the deviation from centerline from your controller, but I doubt in practice this is something anyone cares about. As long as you are within the 8 miles protected width of the airway, I would be surprised if any controller would complain, never mind would the FAA file an enforcement action. As far as what airways are "all about", clearly they are not all about making ATC's job easier. That's part of it of course, but VOR airways would be useful even if ATC didn't exist.

@asb1230: see e.g. https://en.wikipedia.org/wiki/Strategic_lateral_offset_procedure, https://www.pilotsofamerica.com/community/threads/can-i-fly-off-center-on-a-victor-airway.90650/

Great references! I am understanding that despite the "keep them together to keep them apart" idea of airways, GPS navigation can backfire and "keep them too together" so to speak.